Light sources are known per se. They are used, inter alia, in all kinds of illumination systems, for example, for use as general illumination system in shops and offices, or, for example, for use as illumination system in projection devices such as beamers and projection televisions, or, for example, for use as backlight illumination system in display devices, such as liquid crystal display devices. The light source may also include a luminescent material for determining a color of the light emitted by the light source. Light sources which use luminescent material for determining the color of the emitted light comprise light sources having a light emitter which emits non-visible light and a light emitter which emits visible light.
In the known embodiment of the light source having a light emitter which emits non-visible light, the non-visible light is substantially completely converted into visible light by the luminescent material which determines the color of the visible light emitted by the light source. Such a light emitter, for example, emits ultraviolet light, which is converted by a luminescent material into visible light. The luminescent material typically comprises a mixture of luminescent materials, which mixture of luminescent materials determines the color of the visible light emitted by the light source. Examples of such light sources having a light emitter which emits non-visible light are, for example, low pressure mercury vapor discharge lamps which emit ultraviolet light towards the luminescent layer, which converts the ultraviolet light into visible light.
In the known embodiment of the light source which comprises a light emitter which emits visible light, only a part of the light from the light emitter is converted by the luminescent material and the remainder of the light from the light emitter is combined with the converted light to determine the color of the visible light emitted by the light source. Again, the luminescent material may comprise a mixture of different luminescent materials to tune the color of the light emitted by the light source. A major benefit of this embodiment is that the efficiency of the light emitted by the light source is increased due to the reduction of the light conversion. When luminescent materials are used to convert light from one color into a further color, loss of light occurs due to the Stokes shift. When using part of the light emitted by the light emitter, this light need not be converted into a different color, thus reducing the loss due to the Stokes shift and improving the efficiency of the light source. Especially due to the development of Blue LEDs and Blue Laser-LEDs, these types of light sources having a light emitter which emits visible light have become very popular.
However, a drawback of the known light source having a light emitter emitting visible light, is that generally the light source does not have the same color appearance during non-operation as during operation. This results from the configuration of the light source being such that the light emitted by the light emitter is emitted away from the light source via the luminescent material, which generally is applied as a luminescent layer on top of the light emitter. As a consequence, the appearance of the light source is mainly determined by the appearance of the luminescent material. For example, when the light source has a light emitter which emits blue light via a luminescent layer which converts part of this blue light into yellow light, the light source emits substantially white light during operation, while the appearance of the light source during non-operation is substantially yellow due to the luminescent material.
A solution to this difference in appearance during operation and non-operation may, for example, be found in US patent application US 2005/0201109. In this patent application a lighting apparatus is disclosed which comprises a support, a light source having an emission surface and, disposed in the support, a lens disposed to face the emission surface of the light source. A half-mirror film is provided on at least a surface of the lens. The half-mirror film prevents viewing the inside of the lighting apparatus from the outside using external light when light is not being emitted from the light source.
A drawback of this known lighting apparatus is that such a half-mirror film can only be used efficiently in a relatively limited wavelength range. When substantially all external light should be blocked from entering the light source, the half-mirror film will also block part of the light from being emitted from the lighting apparatus, thus reducing the efficiency of the known lighting apparatus significantly.